This invention relates to a piston for a reciprocating machine and more particularly to an improved material and process for forming such a piston.
In reciprocating machines and particularly those like an engine, be they of the two or four cycle type, the piston is subject to quite high loading and conditions which provide substantial but different stresses thereon. That is, a conventional engine piston has a piston head which faces and defines in part the combustion chamber. This piston head may be formed with one or more piston ring grooves for sealing purposes with the associated cylinder bore. In addition, the piston has a skirt portion that provides slidable engagement with the cylinder bore.
It is necessary that the head of the piston have a very high heat resistance due to its exposure to the combustion chamber. The skirt portion, on the other hand, should have very high abrasion resistance and strength to prevent cracking or breaking.
In addition to these stress considerations, it is also desirable to have the piston be light in weight and to have a relatively thin wall construction, particularly in the skirt area, to accomplish this light weight. The light weight obviously reduces the reciprocating forces and the amount of weight that must be balanced. However, the material should also be high in fatigue strength, particularly under high temperatures and provide high abrasion resistance.
It has been the general practice to form the pistons for engines and other reciprocating machines from aluminum (Al) or an aluminum alloy. Silicon (Si) is employed as an alloying material for increasing abrasion resistance and resistance to cracking. Copper (Cu) and magnesium (Mg) have also been employed as alloying materials to increase the strength. It has also been proposed to add a material such as a ceramic fiber like aluminum oxide (AL.sub.2 O.sub.3) or silicon carbide (SiC) which are harder components than the silicon for improving the abrasion resistance in at least the skirt area.
Normally, the piston is formed by a casting process. A difficulty with the casting process for forming the piston is that the molding of the piston and subsequent solidification causes the solidified texture of the metal to become relatively coarse and thus reduce its strength and creates brittleness. Forging can avoid these tendencies.
Iron (Fe) is also frequently added to increase the abrasion resistance and fatigue strength of the finished piston. However, like the use of silicon and silicon carbides the utilization of these alloying materials in a casting process gives problems upon solidification. This is primarily due to the fact that the metal ingredients have different melting points and the molding process does not assure uniformity in the material dispersion nor in the crystal size. Forging can also avoid these tendencies
If, however, silicon is employed in the alloy, then forming the piston by forging presents certain other problems. Normally, the silicon employed in the piston formation has a primary silicon crystal configuration of greater than 10 .mu.m in size. As a result, thy is a likelihood that the primary crystal silicon particles in the skirt portion will become fractured when forging. This can cause cracks to be formed in the boundaries between the silicon particles and the remainder of the matrix. This reduces the fatigue strength of the skirt portion substantially.
It is, therefore, a still further object of this invention to provide an improved method and material for forming a piston by forging utilizing a powdered metal process having the desired alloy characteristics.